358 Mr. N. Colgan oji Locomotion and the 



The slime-thread in this, as in all the species observed, 

 was distinctly elastic. When one of the floating animals 

 was gently puslied outwards by a needle-point from the side 

 of the glass tube b)' which it had ascended, it would spring 

 backwards towards the side as soon as the needle was with- 

 drawn. The thread or film was evidently continuous along 

 the side of the tube and over the water surface to the point 

 where the animal floated. This was more than once made 

 apparent in this way: — An individual floating quite close to 

 the side of the tube would drop and suddenly come to rest 

 about half an inch below the water surface and against the 

 f-ide of the tube. If tha tube were then quickly moved from 

 a vertical to an almost horizontal position the animal would 

 be found hanging suspended across the tube from a point in 

 the side. This suspension was evidently from a portion of 

 the slirae-thread formed by the animal in ascending, as the 

 change of position of the tube was effected so quickly as to 

 prevent the animal applying its foot afresh to the glass 

 surface, and so producing anew attachment and anew thread. 



The peculiar jerky method of descent by its slirae-thread 

 frequently observed to take place with this species appears to 

 me to negative the idea that such motion is accidental or 

 involuntary, as has been suggested by G. SheritF Tye * and 

 H. Wallis Kew f in their well-known papers on the subject 

 of thread-spinning in the Mollusca. The abrupt pauses in 

 and resumptions of the downward motion of Skenea appear 

 to me to be explicable only on the assumption that the animal 

 while descending, voluntarily and at intervals, inhibits and 

 sets in action the discharge of mucus — in other words, that 

 it makes its suspensory thread of set purpose. The appear- 

 ances are inconsistent with an accidental lengthening of an 

 elastic film, caused by the animal suddenly losing its foothold 

 on the water surface, and so throwing its weight on that film 

 at one point. 



Rissoa striata (Adams). — This rather sluggish species, 

 as it proved to be, was observed several times on the 1st May 

 last to drop from the water surface and hang suspended by 

 its thread. On one occasion the thread was seen to issue 

 from the edge of the closed operculum, as if the animal had 

 released itself from the water surface on which it crawled by 

 withdrawing its foot. One individual was seen to mount its 

 thread for tliree-quarters of an inch in two minutes, the 



* Quarterly Journ. of Concliol. vol. i. p. 402 (MoUuscan Threads). 

 t ' Zoologist,' no. 709, Jiilj 1900 (Spinuing Molluscs). 



